THE PRODUCTION OF ULTRAFINE SILICA POWDERS FROM SILICON TETRACHLORIDE- CONTROL OF THE PRIMARY PARTICLE-SIZE

Ultra fine silica powders were prepared by hydrolysis of SiCl4 using a queous ammonia solution followed by supercritical drying. Using differ ent methods of combining the SiCl4 and ammonia solution, to vary the i nitial and final pH of the solution, large silica powders surface area s (271-905 m(2)/g), fine average particle diameters (3.5-17) nm and lo w tapping densities (0.02-0.05 g/cm(2)) could be prepared. Powders wit h characteristics similar to pyrogenic silica, and with similar therma l stability at temperatures up to 1000 degrees C, could be produced.